Liquid scintillation has been mainly employed for measurement of radiation from a low-energy beta emitter (for example, .sup.14 C, .sup.3 H) contained in an organic sample such as a biological sample, etc. The liquid scintillation wherein the sample itself is dissolved or dispersed in the liquid scintillation medium can measure even low-energy beta radiation with high sensitivity.
In an experiment of tracer for biological tissues, etc., the sample is frozen to prevent migration of water-soluble or volatile labeled substance (elution from the sample, diffusion in the sample, etc.) and a thin section is prepared while the sample is frozen, which is directly lyophilized in a vacuum and its radioactivity is measured.
In the known liquid scintillation method, however, the preparation of the scintillator solution containing radioactive samples is troublesome and the solution should be treated after radiation measurement to prevent environmental contamination.
For treatment of the scintillator solution after measurement, there are two methods, that is, a method wherein the solution is directly incinerated and a method wherein only the combustibles are incinerated after pretreatment. The former has serious problems, that is, the presence of noncombustibles, scattering of radioactivity, etc. which are obstructions to practical application.
As the pretreatment, a method has been employed wherein the waste solution is distilled and the distillate, optionally combined with the residual solution after filtration, and incinerated with liquid fuel such as kerosene. The method, however, requires a complicated and expensive apparatus.
As other defects of the liquid scintillation method, a large number of substances which act as quenchers for the scintillator (hereinafter simply referred to as quencher) exist in the biological sample which contains a large amount of biological components other than the labeled substances, making for complicated effects. Accordingly, it is difficult to quantitatively measure a very small amount of radioactivity by the liquid scintillation method. The effect of such quenchers may be removed to some extent by compensation for the quenching which requires an additional apparatus, labor and time.
The method of lyophilizing the sample, wherein the preparation of the sample section and vacuum lyophilization are carried out at low temperature, requires a large-scale equipment, much labor and time.